High strength friction stir welding

Abstract

A method for reducing material property degradation during friction stir welding. More specifically, the method includes the steps of solution heat treating first and second structural members at a first predetermined temperature schedule. The first and second structural members are then quenched to a predetermined temperature at which the structural members are in a nonequilibrium state and have an incomplete temper. The first structural member is then positioned adjacent to the second structural member, thereby defining an interface therebetween. Thereafter, the first and second structural members are joined to form a structural assembly by friction stir welding the material along the interface prior to precipitation heat treating the structural assembly. The structural assembly is then aged, such as by precipitation heat treating, at a second predetermined temperature schedule to stabilize the material properties of the resulting structural assembly, thereby completing the temper of the material. The method requires fewer manufacturing steps than conventional techniques for friction stir weld precipitation-hardened parent materials. In addition, the method minimizes the degradation of the material properties during friction stir welding, and produces a structural assembly with improved strength, hardness, and corrosion resistance, as well as dimensional quality.

That which is claimed is:  
     
       1. A method for reducing material property degradation during friction stir welding, the method comprising the steps of: 
       solution heat treating first and second structural members at a first predetermined temperature schedule;  
       quenching the first and second structural members following said solution heat treating step to a predetermined temperature at which the first and second structural members are in a nonequilibrium state;  
       positioning the first structural member adjacent to the second structural member following said quenching step to thereby define an interface therebetween; and  
       thereafter, joining the first and second structural members to form a structural assembly by friction stir welding the first and second structural members along the interface prior to precipitation heat treating the resulting structural assembly to stabilize the material properties of the structural assembly.  
     
     
       2. A method as defined in claim  1 , further comprising the step of machining the first and second structural members to a corresponding preselected shape and thickness prior to said positioning step. 
     
     
       3. A method as defined in claim  1 , further comprising the step of further quenching the first and second structural members simultaneously with said joining step to reduce the size of a heat-affected region in the first and second structural members created by said friction stir welding. 
     
     
       4. A method as defined in claim  3  wherein said further quenching step comprises applying a liquid coolant to the first and second structural members using at least one liquid coolant jet. 
     
     
       5. A method as defined in claim  3  wherein said further quenching step comprises applying a gaseous coolant to the first and second structural members using at least one gaseous coolant jet. 
     
     
       6. A method as defined in claim  3  wherein said further quenching step comprises applying a liquid coolant to the first and second structural members by submersing the first and second structural members in a liquid coolant bath. 
     
     
       7. A method as defined in claim  3 , further comprising the step of precipitation heat treating the structural assembly by aging the structural assembly at a second predetermined temperature schedule. 
     
     
       8. A method as defined in claim  1 , further comprising the step of cooling the structural assembly to ambient temperature. 
     
     
       9. A method as defined in claim  8 , further comprising the step of securing the structural assembly following said cooling step to other structural assemblies to form the frame of an aircraft. 
     
     
       10. A method for reducing material property degradation during friction stir welding, the method comprising the steps of: 
       solution heat treating first and second structural members at a first predetermined temperature schedule;  
       quenching the first and second structural members following said solution heat treating step to a predetermined temperature, wherein said solution heat treating and quenching steps are conducted such that the resulting structural members have an incomplete temper;  
       positioning the first structural member adjacent to the second structural member following said quenching step to thereby define an interface therebetween; and  
       thereafter, joining the first and second structural members to form a structural assembly by friction stir welding the first and second structural members along the interface even though the first and second structural members have an incomplete temper.  
     
     
       11. A method as defined in claim  10 , further comprising the step of machining the first and second structural members to a corresponding preselected shape and thickness prior to said positioning step. 
     
     
       12. A method as defined in claim  10 , further comprising the step of further quenching the first and second structural members simultaneously with said joining step to reduce the size of a heat-affected region in the first and second structural members created by said friction stir welding. 
     
     
       13. A method as defined in claim  12  wherein said further quenching step comprises applying a liquid coolant to the first and second structural members using at least one liquid coolant jet. 
     
     
       14. A method as defined in claim  12  wherein said further quenching step comprises applying a gaseous coolant to the first and second structural members using at least one gaseous coolant jet. 
     
     
       15. A method as defined in claim  12  wherein said further quenching step comprises applying a liquid coolant to the first and second structural members by submersing the first and second structural members in a liquid coolant bath. 
     
     
       16. A method as defined in claim  12 , further comprising the step of precipitation heat treating the structural assembly by aging the structural assembly at a second predetermined temperature schedule. 
     
     
       17. A method as defined in claim  10 , further comprising the step of cooling the structural assembly to ambient temperature. 
     
     
       18. A method as defined in claim  17 , further comprising the step of securing the structural assembly following said cooling step to other structural assemblies to form the frame of an aircraft.